The invention relates to an amplifier comprising a cascade circuit including an input circuit, a cascode circuit and an output circuit, the input circuit receiving input signals via an input, and the cascode circuit converting the input signals to amplified signals, the output circuit supplying the amplified signals to an output, and the cascode circuit comprising an input transistor and an output transistor.
The invention also relates to a CATV amplifier module comprising a cascade circuit including an input circuit, a cascode circuit and an output circuit, the input circuit receiving input signals via an input, and the cascode circuit converting the input signals to amplified signals, the output circuit supplying the amplified signals to an output, and the cascode circuit comprising an input transistor and an output transistor.
The invention further relates to a cascode circuit for amplifying input signals, comprising an input transistor and an output transistor.
Such an amplifier is well-known and applied, inter alia, in the CATV amplifier module BGD902 developed by PHILIPS. Such CATV amplifier modules are employed in coaxial cable networks to maintain the signal level. For this purpose, these CATV amplifier modules comprise a cascade circuit including, respectively, an input circuit, an amplifier circuit and an output circuit. As these CATV amplifier modules must meet very high distortion requirements, the amplifier circuits are generally embodied so as to be symmetrical push-pull amplifiers. By virtue of this measure, the second-order products formed cancel each other out in the output circuit. In this respect, it is very important that both amplifier branches should be symmetrically driven and loaded. In the known CATV amplifier module, two cascode circuits, accommodated in a symmetrical push-pull configuration, are responsible for amplifying the input signals. It is to be noted that the amplifier in accordance with the invention does not necessarily comprise a plurality of cascode circuits accommodated in a symmetrical configuration. The amplifier in accordance with the invention may alternatively comprise only one cascode circuit.
The known cascode circuit is responsible for a comparatively high intermodulation distortion contribution to the amplified signals.
It is an object of the invention to provide an amplifier of the type mentioned in the opening paragraph, in which the cascode circuit""s contribution to the intermodulation distortion in the amplified signals is comparatively small.
To achieve this, the amplifier in accordance with the invention is characterized in that the cascode circuit comprises a further input transistor, the input transistor and said further input transistor being parallel-connected for comparatively high-frequency input signals, and the input transistor and said further input transistor being series-connected for comparatively low-frequency supply signals. The invention is based on the recognition that the comparatively high contribution to the intermodulation distortion by the known cascode circuit is caused predominantly by the input transistor. This can be attributed to the non-linear relation between the collector current and the base-emitter voltage (for a bipolar input transistor) or between the drain current and the gate-source voltage (for a field effect input transistor). It is well known that the influence of these non-linearities can be reduced by increasing the transconductance of the input transistor.
The transconductance of a bipolar transistor can be readily increased by increasing the DC collector current Ic (a supply signal). Likewise, the transconductance of a field effect transistor can be readily increased by increasing the DC drain current Id (a supply signal). However, for many amplifiers, including the above-mentioned CATV amplifier modules, this does not constitute a workable alternative because the amplifier""s power dissipation is subject to strict limitations.
Providing the cascode circuit with an additional input transistor, which, for comparatively high-frequency (for example RF) input signals, is parallel-connected to the input transistor already present, and which is series-connected, for comparatively low-frequency (for example DC) supply signals, to said input transistor already present, results in the transconductance being doubled, while the DC collector current or the drain current remain the same. By doubling the transconductance, the contribution of the cascode circuit to the intermodulation distortion in the amplified signals is reduced substantially. As the DC collector current or drain current remains unchanged, the power dissipated in the amplifier remains substantially the same.
It is to be noted that United States patent specification U.S. Pat. No. 4,590,436 discloses an amplifier circuit wherein two transistors are parallel-connected for AC input signals and said transistors are series-connected for DC input signals. As a result of the DC series-connection of the transistors, only half the supply voltage is applied across the two transistors. By virtue thereof, the supply voltage is not limited to a value equal to half the breakdown voltage of the individual transistors. The use of a higher supply voltage will lead, however, to a higher power dissipation in this known amplifier. A further difference between this known amplifier and the amplifier in accordance with the invention resides in that the known amplifier does not comprise a cascode circuit. Besides, in said United States patent specification no mention is made of influences of the known amplifier on the intermodulation distortion contribution in the amplified signals.
An embodiment of the amplifier in accordance with the invention is characterized in that the input transistor receives the input signals on a control electrode, and the output transistor supplies the amplified signals on a first main electrode, a second main electrode of the output transistor being connected to a first main electrode of the input transistor, and the control electrode being connected via a first capacitor to a further control electrode of the further input transistor, and the first main electrode of the input transistor being connected via a second capacitor to a further first main electrode of the further input transistor, a second main electrode of the input transistor being connected via a third capacitor to a further second main electrode of the further input transistor, and the second main electrode of the input transistor being connected via a coil to the further first main electrode of the further input transistor. By virtue of the fact that all corresponding electrodes of the input transistors are connected to each other via capacitors, it is achieved in a simple way that the input transistors for comparatively high-frequency input signals are parallel-connected. In addition, the coil, which is preferably embodied so as to be a choke coil having a core of a soft-magnetic material (for example ferrite), ensures that the input transistors for comparatively low-frequency supply signals are series-connected.